LT1110 Linear Technology, LT1110 Datasheet - Page 11

• Manufacturer Part Number: LT1110
• Description: Micropower DC-DC Converter Adjustable and Fixed 5V/ 12V
• Manufacturer: Linear Technology

dissipation inside the LT1110. Output voltage is deter-
mined by
R3 programs switch current limit. This is especially im-
portant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time
each cycle. Under certain conditions the current in L1 can
build up to excessive levels, exceeding the switch rating
and/or saturating the inductor. The 220 resistor pro-
grams the switch to turn off when the current reaches
approximately 800mA. When using the LT1110 in step-
down mode, output voltage should be limited to 6.2V or
less. Higher output voltages can be accommodated by
inserting a 1N5818 diode in series with the SW2 pin
(anode connected to SW2).
Higher Current Step-Down Operation
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 6. R1 serves as a
current limit sense. When the voltage drop across R1
equals a V
pensation a Schottky diode can be inserted in series with
the I
to V
current is limited to 2A. Inductor value can be calculated
based on formulas in the “Inductor Selection Step-Down
A
MAX
25V
PPLICATI
V
V
IN
LIM
C2
OUT
BE
Figure 6. Q1 Permits Higher-Current Switching.
LT1110 Functions as Controller.
pin. This also lowers the maximum drop across R1
– V
+
BE
D
, the switch turns off. For temperature com-
1
, increasing efficiency. As shown, switch
GND
V
IN
R
LT1110
R
O
0.3
2
1
R1
U
SW2
I
S
L
220
SW1
FB
R2
220
I FOR ATIO
mV
U
ZETEX ZTX789A
330
R3
MJE210 OR
.
R5
Q1
R4
W
V
D1
1N5821
OUT
L1
= 220mV
LT1110 • TA16
U
(
+
1 +
C1
V
OUT
R4
R5
( )
)
23
Converter” section with the following conservative ex-
pression for V
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage.
Inverting Configurations
The LT1110 can be configured as a positive-to-negative
converter (Figure 7), or a negative-to-positive converter
(Figure 8). In Figure 7, the arrangement is very similar to
a step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and V
put voltages can be accommodated as in the prior section.
In Figure 8, the input is negative while the output is
positive. In this configuration, the magnitude of the input
voltage can be higher or lower than the output voltage. A
level shift, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.
–V
+V
IN
V
IN
+
SW
+
OUT
C2
C2
Figure 7. Positive-to-Negative Converter
Figure 8. Negative-to-Positive Converter
V
AO
should be less than 6.2V. More negative out-
R
GND
I
I
LIM
LIM
SW
1
LT1110
R3
:
LT1110
V
GND
V
V
SW2
SAT
IN
IN
SW1
FB
SW2
SW1
FB
L1
0 9
.
V
.
R2
D1
1N5818
L1
D1
V
OUT
+
+
=
( )
C1
R1
R2
C1
220mV + 0.6V
LT1110
2N3906
R1
R2
R1
LT1110 • TA03
11
LT1110 • TA04
–V
+V
( )
OUT
24
OUT